With the recent advancement in the field of high density information storage device, it has become a matter of special interest to develop an optical or electronic memory device through the implementation of a chalcogenide-based phase change material, which is advantageous in that it undergoes fast and reversible phase changes between the crystalline and amorphous states. By exploiting the differences between the crystalline and amorphous states in, e.g., optical reflectance, transmittance and electrical resistance, information storage can be achieved. For instance, the difference in optical characteristics is employed in rewritable CD and DVD optical disks, while non-volatile phase change memory and electrical probe-based memory exploit the difference in the electrical resistance between crystalline and amorphous states.
Specifically, non-volatile phase change memory devices named alternatively OUM (Ovonic Unified Memory), PRAM (Phase-change Random Access Memory), or CRAM (Chalcogenide Random Access Memory) have been actively researched for commercialization as viable substitutes for DRAM.
In FIGS. 1a and 1b, two kinds of non-volatile phase change memory devices are presented.
Basically, they both have top electrodes 11, 11′ and bottom electrodes 12, 12′ for the input and output of electrical signals; memory areas 13, 13′ containing chalcogenide-based phase change materials; and insulating areas 14, 14′ for electrical and thermal insulation.
The difference between these two structures is that the memory device of FIG. 1b is further provided with a separate electrode 15′ for the joule-heating of the memory area 13′. On the other hand, the memory material itself is responsible for the joule-heating in FIG. 1a, the memory material being disposed in the central enclosed area 13.
When an electric voltage or current pulse is applied between the top and bottom electrodes, there takes place direct or indirect heating to melt the phase change material. At the end of the electric pulse, the melted phase change material is quenched in the form of an amorphous state, achieving information writing. This operation is called a reset operation. To erase this stored information, the electric pulse is applied to the amorphous phase change material such that suitable crystallization conditions in terms of, e.g. heating time and temperature, are met. Once crystallized, the stored information in the memory cell is erased. This operation is called a set operation.
The memory cell presents different electrical resistance depending on whether it is in a crystallized or an amorphous state. The amorphous state presents higher electrical resistance than the crystallized state. Therefore, by sensing the electrical resistance of a memory cell, stored binary information can be read.
Stoichiometric composition of a GeSbTe-based alloy was developed as a practical phase change material capable of electrically switching between the amorphous and crystallized states in a reversible manner. Despite its merits as a non-volatile phase change memory material, the GeSbTe based alloy is disadvantageous in that it has a relatively high melting point of 600° C.˜700° C. and possibly a high crystallization temperature (the crystallization temperature is generally ½˜⅔ of the melting point on an absolute temperature scale). This is problematic since such a high melting point and possibly a high crystallization temperature, requires more current and power for a reset and a set operation of a phase change memory cell.
Shown in FIG. 2 is the dependency of the current level supplied from a transistor and that needed for a reset operation on the feature size(F) or contact size(0.5 F). The dashed line in FIG. 2 indicates the current level of a transistor supplied to a phase change memory cell having 8 F2 DRAM structure as schematically shown in the incorporated figure. The solid line in FIG. 2 represents the change in the current level needed for a reset operation when the current density level is maintained at 100 mA/μm2, which is the value slightly smaller than the current density values estimated from the reset current levels and contact sizes of existing phase change memory prototypes (refer to the results of Intel/Ovonyx in ISSCC 2002 and Samsung Electronics in NVSMW 2003, the current density being 123.5 mA/μm2 and 138.9 mA/μm2, respectively).
As shown in FIG. 2, the current level supplied from the transistor cannot match the required reset current level until the feature size is reduced down to about 45 nm or smaller.
There might be two options in reducing the current level needed for a reset operation: lowering the melting point of the phase change material used in the memory cell, or enhancing generation/confinement of joule heat by way of changing the materials and the structure of the memory cell.
However, dramatic improvement cannot be expected just by lowering the melting point of the memory material since the melting point of, e.g., 900 K needs to be cut down to the level of about half (450 K) just to save 50% of the electric power. Accordingly, a practical approach would be to look for an optional material/structure for other parts of the memory cell, thereby optimizing heating and thermal dissipation characteristics of the memory material during reset/set process.
To establish an electrical current path in the phase change memory device, the memory area must be in contact with electrodes. Electrodes may be connected directly to the memory area or through electrode contact layers. As shown in FIGS. 3a and 3b, electrode contact layers 26, 27, 26′, 27′ are disposed between memory materials 23, 23′ and electrodes 21, 21′, 22, 22′, 25′ to block material diffusion therebetween.
Amorphous carbon, amorphous silicon or amorphous C/Si double structure was employed as an electrode contact layer in Ovonic EEPROM (Electrically Erasable Programmable Read Only Memory). It was also proposed to pile carbon and molybdenium layers to use as an electrode contact layer. Another exemplary material for electrode contact layer is combinations of: the material selected from the group consisting of Ti, V, Cr, Nb, Mo, Ha, Ta, W and mixtures or alloys thereof; and two or more elements selected from the group consisting of B, C, N, O, Al, Si, P and S. Examples of Such combination encompass TiCN, TiAlN TiSiN, W—Al2O3 and Cr—Al2O3.
Heat dissipation from the memory area is affected by such factors as what constitutes electrode/electrode contact layers and how memory area is insulated from surroundings. Materials such as Al, Cu, Ti, Mo, W, Poly-Si, TiW, TiN and TiAlN are typically employed as electrodes/electrode contact layers. Electrical conductivities of these materials are relatively high 103˜106 Ω−1cm−1. However, these materials also have a high thermal conductivity, almost several tens or hundreds of times higher than that of a GeSbTe alloy material or an insulating material for a memory device. Accordingly, phase change memory devices employing these materials as electrodes tend to have large heat dissipation during the reset/set process and, as a result, more power is needed to elevate the temperature of the memory area over the melting point or crystallization temperature thereof.
Further, mutual diffusion might happen between the memory material and the above mentioned electrodes/electrode contact materials when heated above melting/crystallization temperature. Under the circumstance, it is hard to expect that such a memory device would function in a stable manner Accordingly, there existed a need to find new materials for use in electrodes/electrode contact layers that will lessen power consumption and improve operational stability of the memory device.